Plumbing question - combination valve

[turtlewaxer]

Hi, I've got a loud noise like a screechy fog horn whenever the cold water tap is ran wide open or toilet is flushed. Some internet searching suggests it could be the combination valve. I have a reliance 3 bar reducing 5 bar relief inlet control multibloc 2 piece valve fitted next to cylinder. I have put a screw on pressure gauge on the outside tap which shows 6.5 to 7 bar so my question is the combination above sized correctly or should I have a slightly larger relief valve?